Electroluminescent materials

ABSTRACT

An electroluminescent material which can form electroluminescent devices which emit light in the ultra-violet region of the spectrum which comprises an organic metallic complex of a transition metal, lanthanide or actinide and a polyamine ligand particularly gadolinium ethylenediaminetetraacetate sodium salt, Na[Gd(EDTA)] and gadolinium europium ethylenediaminetetraacetate salts Gd[Eu(EDTA)] 3 .

This application is a national stage application of PCT/GB00/00268 whichwas published in English under publication number WO 00/44851 oil Aug.3, 2001.

The present invention relates to electroluminescent materials which canemit light in the ultra-violet region of the spectrum and devices madeusing such materials.

BACKGROUND OF THE INVENTION

Materials which emit light when an electric current is passed throughthem are well known and used in a wide range of display applications andorganic polymers have been proposed as useful in electroluminescentdevices, but using these polymers it is not possible to obtain purecolours, they are expensive to make and have a relatively lowefficiency.

Another compound which has been proposed is aluminium quinolate, butthis requires dopants to be used to obtain a range of colours and has arelatively low efficiency.

In an article in Chemistry letters pp 657-660, 1990 Kido et al disclosedthat a terbium (III) acetyl acetonate complex was greenelectroluminescent and in an article in Applied Physics letters 65 (17)Oct. 24, 1994 Kido et al disclosed that a europium (III) triphenylenediamine complexes was red electroluminescent but these were unstable inatmospheric conditions and difficult to produce as films.

The complexes disclosed in these articles had a relatively lowphotoluminescent efficiency and were only able to produce green or redlight and other colours could not be produced.

Patent Applications WO 98/58037 and WO 98/55561 discloseelectroluminescent materials which emit light in the visible spectrumand disclose electroluminescent devices incorporating these materials.

The wavelength of the emitted light from an organo metallic complexwhich is an electroluminescent material depends on the metal or metalsand the ligands.

In general the shorter the wavelength of the emitted light the harder itis to obtain effective electroluminescent materials and in particularelectroluminescent materials which emit light in the ultra-violet regionof the spectrum have hitherto not been possible to produce.

An electroluminescent material which can emit ultra-violet light wouldhave a range of applications and would enable there to be a source ofultra-violet light without the need for high temperature or complex highenergy or vacuum equipment and so could replace such sources ofultra-violet light.

For example, there are devices and displays etc. where ultra-violetlight is used to excite other materials to cause these other materialsto fluoresce in the visible spectrum.

For example, there are devices and displays etc. where ultra-violetlight is used to excite other materials to cause these other materialsto fluoresce in the visible spectrum.

SUMMARY OF THE INVENTION

We have now devised organo-metallic complexes which can emitultra-violet light.

According to the invention there is provided an electroluminescentmaterial which emits light in the ultra-violet region of the spectrumwhich comprises an organic metallic complex of a transition metal,lanthanide or actinide and a polyamine ligand.

The preferred metal is gadolinium in the III state.

Preferred ligands are ethylene diamine tetramine EDTA, DCTA, DTPA andTTHA.

The structural formulae of these compounds in the acetic acid form areshown below.

The organo metallic complexes can be made by the reaction of gadoliniumchloride with an alkali metal salt of the amine e.g. the sodium salt,for EDTA the reaction is

GdCl₃+Na₄(EDTA)→Na⁺[Gd(EDTA)]⁻

For the other polyamine the corresponding alkali metal salt is used.With polyamines which arc more than tri-functional e.g. EDTA, DCTA,DTPA, TTHA the complexes can be in the form of a salt e.g. an alkalimetal salt and can be used in this form. Alternatively a transitionmetal, lanthanide or actinide salt e.g. Ln*[Ln(EDTA)]₃ where Ln and Ln*is a transition metal, lanthanide or actinide and preferably alanthanide e.g. Gd, Sm, Eu, Tb, Dy, etc.

Particularly preferred mixed complexes are the gadolinium complexes e.g.Gd[Eu(EDTA)]₃.

It has surprisingly been found that the lanthanide salts of theorgano-metallic complexes exhibit a different electroluminescentspectrum than mixed organ-metallic complexes.

The mixed lanthanide salts can be made by reacting a lanthanide chloridewith a lanthanide polyamine complex.

The materials of the present invention can be incorporated intoelectroluminescent devices which emit ultra-violet light and theinvention include such electroluminescent devices.

The materials of the present invention can be incorporated intoelectroluminescenlt devices which emit ultra-violet light and theinvention include such electroluminescent devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a spectrum of gadolinium ethylenediaminetetracetate sodiumsalt; and

FIG. 2 is a spectrum of gadolinium europium ethylenediametetracetatesalt.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ultra-violet light emitting devices of the invention comprise atransparent substrate which is a conductive glass or plastic materialwhich acts as the anode, preferred substrates arc conductive glassessuch as indium tin oxide coated glass, but any glass which is conductiveor has a conductive layer can be used. Conductive polymers andconductive polymer coated glass or plastics materials can also be usedas the substrate. The electroluminescent material can be deposited onthe substrate directly by evaporation from a solution of the material inan organic solvent. The solvent which is used will depend the materialfor example alcohols such as ethanol, ketones such as acetone and methylacetylacetonate and chlorinated hydrocarbons such as dichloromethane aresuitable in many cases.

Alternatively the material can be deposited by spin coating or by vacuumdeposition from the solid state e.g. by sputtering or any otherconventional method can be used.

In one embodiment there is a hole transporting layer deposited on thetransparent substrate and the electroluminescent material is depositedon the hole transporting layer. The hole transporting layer serves totransport; holes and to block the electrons, thus preventing electronsfrom moving into the electrode without recombining with holes. Therecombination of carriers therefore mainly takes place in the emitterlayer.

Hole transporting layers are used in polymer electroluminescent, devicesand any of the known hole transporting materials in film form can beused.

The hole transporting layer can be made of a film of an aromatic aminecomplex such as poly(vinylcarbazole),N,N′-diphenyl-N,N′-bis(3-methylphenyl)-I,I′-biphenyl-4,4′-diamine (TPD),polyaniline etc.

Optionally dyes such as fluorescent laser dyes, luminescent laser dyescan be included so that these dyes fluoresce in the ultra-violet lightto give emitted light of a particular colour spectrum.

Preferably the electroluminescent material is mixed with a polymericmaterial such as a polyolefin e.g. polyethylene, polypropylene etc. andpreferably polystyrene. Preferred amounts of active material in themixture is from 95% to 5% by weight of active material and morepreferably 25 to 20% by weight.

The hole transporting material can optionally be mixed with theelectroluminescent material in a ratio of 5-95% of theelectroluminescent material to 95 to 5% of the hole transportingcompound. In another embodiment of the invention there is a layer of anelectron injecting materials between the cathode and theelectroluminescent material layer., Suitable electron injectingmaterials include a metal complex or oxadiazole or an oxadiozole or anoxadiazole derivative, for example2-(4-biphenyl-5-(4-tert-butylphenyl)-1,3,4oxadiazole. The electroninjecting material is preferably a metal complex such as a metalquinolate e.g. an aluminum quinolate which will transport electrons whenan electric current is passed through it. Alternatively the electroninjecting material can be mixed with the electroluminescent material andco-deposited with it.

In a preferred structure there is a substrate formed of a transparentconductive material which the anode is on which is successivelydeposited a hole transportation layer, the electroluminescent materiallayer and an electron injection layer which is connected to the cathode.The cathode can be any low work function metal, e.g., aluminum, calcium,lithium, silver/magnesium alloys, etc.

There can be a layer or layers containing a fluorescent material in thedevice so that the ultra-violet light emitted will cause the material tofluoresce and emit light of a particular colour spectrum.

The invention is described in the following examples.

EXAMPLE 1

Gadolinium Ethylenediaminetetraacetate Sodium Salt, Na[Gd(EDTA)]

Gadolinium chloride (10 mmol) was dissolved in water (5 ml).Ethylenediaminetetraacetic acid, tetrasodium salt hydrate (10 mmol) wasdissolved in water (10 ml) and added portionwise to the gadoliniumchloride solution. The solution became warm, and after ca. 15 minutes, awhite precipitate was formed. The mixture was left for a further 2hours. The precipitate was filtered off to give a white solid which waswashed with water (2×5 ml) and dried in air to yield Gadoliniumethylenediaminetetraacetate sodium salt, Na[Gd(EDTA)]

EXAMPLE 2

Gadolinium Europium Ethylenediaminetetraacetate Salts, Gd[Eu(EDTA)]₃

Europium ethylenediaminetetraacetate sodium salt Na[Eu(EDTA)] (4.5 mmol)was dissolved in methanol (30 m]). Gadolinium chloride (1.5 mmol) wasdissolved in water (5 ml) and added portionwise to the europiumethylenediaminetetraacctate sodium salt solution. A white precipitatewas immediately formed. The mixture was left for a further 1 hour. Theprecipitate was filtered off to give a white solid which was washed withmethanol (2×10 ml) and dried in air to yield the product Gadoliniumeuropium ethylenediaminetetraacetate salts, Gd[Eu(EDTA)]₃.Phololuminescence was excited using 325 mn line of Liconix 4207 NB,He/Cd laser. The laser power incident at the sample (0.3 mWcm⁻²) wasmeasured by a Liconix 55 PM laser power meter. The radiance calibrationwas carried out using Bentham radiance standard (Bentham SRS8, Lampcurrent 4,000A, calibrated by National Physical laboratories, England.The PL studies were carried out on samples or films. The Complexes ofthe examples were tested and the results shown in the Spectra attachedas FIGS. 1 and 2.

An electroluminescent device constructed with a film of the compounds ofexamples 1 and 2 with an indium coated glass as anode and an aluminiumcathode emitted light in the ultra-violet spectrum when a low voltagewas applied across the film. The light was emitted in the same spectrumranges as in FIGS. 1 and 2.

What is claimed is:
 1. An electroluminescent material which emits lightin the ultra-violet region of the spectrum which comprises an organicmetallic complex having a formula Ln*[Ln(polyamine)]₃ wherein Ln and Ln*are the same or different and are selected from the group consisting oftransition metals, lanthanides and actinide, and wherein the polyamineis selected from the group consisting of ethylene diamine tetramine,DCTA, DTPA and TTHA.
 2. An electroluminescent material as claimed inclaim 1 in which Ln and Ln* are selected from the group consisting ofGd, Sm, Eu, Tb and Dy.
 3. An electroluminescent material as claimed inclaim 1 wherein the complex is Gd[Eu(EDTA)]₃.
 4. An electroluminescentdevice which comprises sequentially (i) a first electrode comprising atransparent conductive substrate (ii) a layer of an electroluminescentmaterial which emits light in the ultra-violet region of the spectrumand which comprises an organic metallic complex selected from the groupconsisting of Gd[Ln(polyamine)]₃ and M[Gd(polyamine)] wherein M is analkali metal and wherein Ln is selected from the group consisting of atransition metal, lanthanide and actinide and (iv) a metal electrode. 5.An electroluminescent device as claimed in claim 4 in which thetransparent substrate is a conductive glass or plastic material whichacts as the anode.
 6. An electroluminescent device as claimed in claim 4in which there is a hole transporting material mixed with theelectroluminescent material in a ratio of 5 to 95% of theelectroluminescent material to 95 to 5% of the hole transportingmaterial.
 7. An electroluminescent device as claimed in claim 4 in whichthe hole transporting material is an aromatic amine complex.
 8. Anelectroluminescent device as claimed in claim 4 in which the holetransporting material is selected from the group consisting ofpoly(vinylcarbazole),N,N′diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)and polyaniline.
 9. An electroluminescent device as claimed in claim 4in which an electron injecting material is mixed with theelectroluminescent material and co-deposited with it.
 10. Anelectroluminescent device as claimed in claim 9 in which the electroninjecting material is selected from the group consisting of a metalcomplex oxadiazole and an oxadiazole derivative.
 11. Anelectroluminescent device as claimed in claim 10 in which the electroninjecting material is selected from the group consisting of an aluminumquinolate and 2-(4biphenyl)-5-(4-tert-butylphenyl)-1,3,4 oxadiazole. 12.An electroluminescent device as in claim 4 in which the metal electrodeincludes at least one selected from the group consisting of aluminumalloy, magnesium alloy, lithium alloy, calcium alloy and magnesiumsilver alloy.
 13. An electroluminescent device as claimed in claim 4 inwhich there is at least one layer which incorporates a dye whichfluoresces in ultra-violet light to give emitted light in the colorspectrum.
 14. An electroluminescent material as claimed in claim 1wherein the organic metallic complex comprises gadolinium in the IIIstate.
 15. An electroluminescent device as claimed in claim 4 whereinthe organic metallic complex comprises gadolinium in the III state. 16.An electroluminescent device as claimed in claim 15 in which the complexis in the form of a salt.
 17. An electroluminescent device as claimed inclaim 15 in which the complex is in the form of an alkali metal salt.18. An electroluminescent device as claimed in claim 4 in which theelectroluminescent material is in the form of the salt of formulaGd[Ln(EDTA)]₃.
 19. An electroluminescent device as claimed in claim 18in which Ln is selected from the group consisting of Gd, Sm, Eu, Tb andDy.
 20. An electroluminescent device as claimed in claim 4 in which thecomplex is Gd[Eu(EDTA)]₃.